Étude et mise au point d'un procédé de traitement des brasques de l'industrie de l'aluminium

The objective of the present dissertation is to answer the existing need of development of a simple, flexible and efficient spent potliners treatment process. After completion of an exhaustive review of processes presented in the literature, the solution developed is based on the separate treatment of cyanides and fluorides in order to keep the process flexible. The process developed presents three steps: (1) Cyanides removal is carried out by thermal destruction instead of liquid oxidation which is difficult when most of the cyanides present in spent potliners are iron-complexed. (2) Leachable fluoride elimination is obtained by thorough fully washing the spent potliners with an acid solution instead of permanently immobilising it in vitrified matrices. (3) The water used for the leaching steps is then cleaned up of its fluoride content by precipitation with calcium salts. The process water can be potentially recycled, since the possibility of adding a recycling loop to the process has been successfully investigated.

The methodology used was the following: Cyanides destruction was studied in an internal circulating fluidized bed at temperatures in the range 700°C to 800°C. For the first time, an apparent kinetic model of order 1 was fitted to the results in order to describe the gradual destruction of cyanides on the small particles used. This type of kinetics is particularly hard to obtain considering the hazardous nature of spent potliners, and so is really welcome by the industry. The present work was, as well, an opportunity to study the anti-agglomeration properties of various additives. Results tend to show that for the additives to have an effect, their particle size must be small, and it can then be used in proportion lower than 10% in the bed.

Fluoride's washing was studied in laboratory conditions. It was shown by using XRD tools, that only NaF salts leach in great quantity, contrarily to cryolite salts. As well, fluoride leaching kinetics were shown to be likely controlled by particles size consideration. For small size sample, leaching is quasi-immediate. A model based on Langmuir isotherm was used to described fluoride adsorption in case where washing is performed using pure water or solution of 120 g/L Na₂SO₄ salts. This model was used to define the washing efficiency of leaching in order to calculate the minimum number of washing steps necessary in order to reduce the concentration of leachable fluoride in solution below regulations. (Abstract shortened by UMI.)